The present invention relates to a method of manufacturing a silicon device fabricated by processing a silicon substrate, such as various sensors used for measuring inertial force caused by acceleration or angular velocity etc., pressure or other various physical quantities, or a fluid device having a passage through which fluid or the like flows, the passage being formed in the silicon substrate.
Conventionally, a cantilever or a mass of free standing structure formed on a silicon substrate, or a hollow structure formed in the silicon substrate etc., is broadly used for sensors for measuring various physical quantities, micro pumps and so on.
FIGS. 15A to 15F are views showing a conventional manufacturing process for fabricating a device having a movable portion on a silicon substrate. According to the manufacturing process, a plate-shaped silicon substrate 32 is prepared at first, as shown in FIG. 15A. Next, as shown in FIG. 15B, a first oxide film 33, which is to be used as a sacrificial layer, is formed on the silicon substrate 32 by means of the CVD technique or the like, and then a first polysilicon film 34, which is to be used as a seed layer, is formed on the oxide film by means of the low pressure CVD technique or the like. After that, as shown in FIG. 15C, a second polysilicon film 35, which is to become a structural part, is formed on the first polysilicon film 34 using an epitaxial reactor. Further, after the second polysilicon film 35 of a desired thickness has been obtained, as shown in FIG. 15D, a second oxide film 36 as the uppermost layer is formed on the second polysilicon film 35 by means of the CVD technique or the like, and then the second oxide film 36 is subjected to a patterning treatment so as to obtain the structural part of the desired shape. The patterned second oxide film 36 is used as a mask for etching the first and second polysilicon films 34,35 which are to become the structural part thereunder. Next, as shown in FIG. 15E, an etching treatment is performed to the first polysilicon film 34 and the second polysilicon film 35 by means of the reactive ion etching technique or the like till the etching reaches the first oxide film 33. Further, as shown in FIG. 15F, a part of the first oxide film 33, which is located under the first polysilicon film 34, is removed by using hydrofluoric acid or the like. In consequence, there is obtained a movable portion which is substantially composed of the first polysilicon film 34 and the second polysilicon film 35.
FIGS. 16A to 16E are views showing a conventional manufacturing process of a structural part having a movable portion, which is disclosed, for example, in pages 52 to 55 of the bulletin of xe2x80x9cThe 8th International Conference on Solid-State sensors and Actuators, and Eurosensors IXxe2x80x9d held in Stockholm on June in 1995. According to this manufacturing process of the structural part, as shown in FIG. 16A, at first, a first oxide film 38 and a photoresist film 37 are formed on a silicon substrate 39 by turns. Next, as shown in FIG. 16B, the first oxide film 38 is patterned using the photolithography technique so that an oxide film mask is formed. Further, as shown in FIG. 16C, the silicon substrate 39 is etched using the oxide film mask, for example, by means of the reactive ion etching technique so that trenches or holes are formed. Following that, as shown in FIG. 16D, in order to protect side surfaces of the trenches or holes, a second oxide film 40 is formed, for example, by means of the CVD technique, and then portions of the second oxide film 40, which exist on the bottoms of the trenches or holes, are removed by means of the reactive ion etching technique. Then, as shown in FIG. 16E, each neighboring trenches or holes are communicated with each other below the structural part by performing an isotropic reactive ion etching treatment to the silicon substrate 39 using another reactive gas. In consequence, the structural part having free standing structure may be obtained.
FIG. 17 is a view showing a conventional etching apparatus, which is disclosed, for example, in pages 653 to 659 of Number 2 in Volume 137 of xe2x80x9cJournal of Electrochemical Societyxe2x80x9d published on February in 1990. As shown in FIG. 17, the etching apparatus is provided with a power source of constant voltage 41, an amperemeter 42, a counter electrode 43, a reference electrode 44 and an etchant vessel 48 containing an etchant 47. In the etching apparatus, after pits with inverted square pyramid shapes have been formed on a surface of a plate-shaped n-type silicon substrate 46 using KOH, a voltage is applied to the silicon substrate 46 while the silicon substrate 46 is immersed in the hydrofluoric acid aqueous solution, with the silicon substrate used a positive electrode. In addition, light 45 is applied to the silicon substrate 46. In consequence, the silicon substrate 46 is etched in the direction depthwise of the substrate. Thus, regularly disposed pores may be formed by means of the etching process.
FIGS. 18A to 18F are views showing a conventional manufacturing process of a silicon device, which is disclosed, for example, in pages 189 to 197 of Volume 3223 of xe2x80x9cProceedings SPIE Micromachining and Microfabrication Process Technology IIIxe2x80x9d published at Austin in Texas (U.S.A.) on September in 1997. According to the manufacturing process of the silicon device, as shown in FIG. 18A, at first, a plate-shaped n-type silicon substrate 49 is prepared. Further, as shown in FIG. 18B, a silicon nitride film 50 is formed on a surface of the silicon substrate 49. Following that, as shown in FIG. 18C, the silicon nitride film 50 is patterned by means of the photolithography technique so that a pattern 51 is formed. Next, as shown in FIG. 18D, pits 52 with inverted triangle shapes are formed on the silicon substrate 49 using KOH. Moreover, the silicon nitride film 50, which has been used as a mask for the etching using KOH, is removed so that the silicon substrate 49 having the pits 52 as shown in FIG. 18E is obtained. Then, as shown in FIG. 18F, a voltage is applied to the silicon substrate 49 while the silicon substrate 49 is immersed in a hydrofluoric acid aqueous solution, with the silicon substrate used a positive electrode. In addition, light is applied to the silicon substrate 49 so that the silicon substrate is etched in the direction depthwise of the substrate. Thus, trenches 54 are formed in the silicon substrate 49.
Meanwhile, FIG. 19 is a view showing a stepwise reaction mechanism when a conventional p-type silicon substrate is etched in the direction depthwise of the substrate by applying a voltage to the p-type silicon substrate while immersing the p-type silicon substrate in an organic solution including a hydrofluoric acid aqueous solution, with the substrate used as a positive electrode. The mechanism is disclosed, for example, in pages 1006 to 1013 of Number 4 in Volume 141 of xe2x80x9cJournal of Electrochemical Societyxe2x80x9d published on April in 1994. At first, a hydrogen atom [H], which is combining with a silicon atom [Si] on the uppermost surface of the silicon substrate as indicated by (A) in FIG. 19, is changed to a hydrogen ion [H+] due to functions of a fluorine ion [Fxe2x88x92] and a hole [h+] so that the bonding between the hydrogen ion and the silicon atom is ruptured while the silicon atom is changed to a silicon radical as indicated by (B). Further, the silicon radical combines with a fluorine atom due to supply of the fluorine ion and an electron [exe2x88x92] as indicated by (C). Then, as indicated by (D) and (E), the same reaction occurs as to another remaining hydrogen atom, in consequence the silicon atom combines with two fluorine atoms. Moreover, as indicated by (F), the other two bonding hands, each of which has combined with another inner silicon atom, also combine with fluorine atoms due to functions of two hydrogen fluoride molecules [HF], respectively, so that the silicon atom dissolves into the organic solution as silicon tetrafluoride [SiF4].
Meanwhile, in the above-mentioned conventional structural part with free standing structure, which is formed of polysilicon, there is such a problem that the mechanical properties and the reliability are inferior to those of a structural part formed of single crystal silicon. Further, in the conventional manufacturing process, it is impossible to thicken the sacrificial layer for fabricating the hollow structure so that it is impossible to sufficiently enlarge the gap between the movable portion (structural part) and the substrate. Therefore, there is such a problem that the movable portion may stick to the bottom surface of the substrate. Further, there is such a problem that because a complicated manufacturing process is required to fabricate the structural part of single crystal silicon having the free standing structure, it is impossible to fabricate the structural part by means of a simple manufacturing process while improving the yield and raising the reliability. Meanwhile, in the conventional process, when the n-type silicon substrate is etched in the hydrofluoric acid aqueous solution, it is only possible to form pores whose diameters are equal to or smaller than 10 xcexcm, or trenches whose widths are 3 xcexcm. Therefore, for example, it is impossible to fabricate a silicon device of three-dimensional structure having a hollow configuration. Moreover, in the conventional process, there is such a problem that when the p-type silicon substrate is etched in the organic solution including the hydrofluoric acid aqueous solution, it is impossible to control the size or position of the portion which is to be etched.
Meanwhile, the present inventors developed and investigated the manufacturing technique of the silicon device in order to solve the above-mentioned problems. Thus, the present inventors announced the results of the investigation under the title of xe2x80x9cFabrication of free standing structure using single step electrochemical etching in hydrofluoric acidxe2x80x9d in xe2x80x9cThe Eleventh Annual International Workshop on Micro Electro Mechanical Systemsxe2x80x9d held at Heidelberg in Germany during the period from Jan. 25-29, 1998. Further, the results of the investigation is disclosed in pages 246 to 250 of the bulletin of the Workshop (IEEE Catalog Number 98CH36176, Pages 246-250). In the announced investigation, it was clarified that the free standing structure could be fabricated by using an n-type of (100) wafer. Hereupon, the condition for fabricating the free standing structure is such that the etching process is initially performed with the current density of 26 mA/cm2 for thirty minutes, and then performed for seven minutes while the current density is increased to 40 mA/cm2, using a hydrofluoric acid aqueous solution of 5%. In consequence, the present inventors succeeded in fabricating a cantilever of free standing structure.
The present invention has been developed to solve the above-mentioned conventional problems, and has an object of providing a method of manufacturing a silicon device, which is capable of forming a structural part using a single crystal silicon, enlarging the gap between the substrate and the structural part sufficiently, fabricating free standing structure by one single step, providing the silicon device having the free standing structure while improving the yield and raising the reliability, and further manufacturing the above-mentioned silicon device by means of a simple manufacturing process.
A method of manufacturing a silicon device according to a first aspect of the present invention, which has been developed to achieve the above-mentioned object, is characterized in that it includes (i) an etching start pattern forming step for forming etching start patterns on a silicon substrate or on a surface of the silicon substrate, (ii) a first etching step for etching the silicon substrate by applying a voltage to the silicon substrate to form etched portions (for example, openings, trenches) that extend in a direction depthwise of the silicon substrate from the etching start patterns while the silicon substrate is immersed in a solution containing fluorine ions, with the silicon substrate used a positive electrode, and (iii) a second etching step for accelerating etching of the silicon substrate by increasing a current flowing through the silicon substrate after the etched portions have reached a predetermined depth, to thereby form a free standing structure composed of a part of the silicon substrate wherein each neighboring etched portions are communicated with each other (namely, etched surfaces are connected) at a location deeper than the predetermined depth.
In the silicon device having a free standing structure, which is manufactured according to the above-mentioned method, the free standing structure is fabricated by means of the etching process by one single step, and the free standing structure is made of single crystal silicon. On the occasion, the depth of the hollow portion can be controlled by varying the time for performing the etching process. In consequence, the structure formed of single crystal silicon, which has excellent mechanical properties, may be easily fabricated by one single step. Further, because the movable portion composed of the structure, may be prevented from sticking to the substrate, the silicon device, which has high reliability, may be cheaply manufactured with good yield using a low-priced manufacturing apparatus. That is, according to the present manufacturing method, because the free standing structure of the manufactured silicon device is composed of single crystal silicon, the obtained device has excellent mechanical properties and high reliability. Further, because the hollow portion below the free standing structure may be formed with a large size, the free standing structure does not stick to the plate-shaped substrate thereunder so that the yield may be highly improved. In addition, according to the present method of manufacturing the silicon device, because the movable portion composed of the free standing structure can be fabricated in one single step, the manufacturing process may be simplified so that the silicon device may be obtained at a low cost.
A method of manufacturing a silicon device according to a second aspect of the present invention is characterized in that it includes (i) an etching start pattern forming step for forming etching start patterns on a silicon substrate or on a surface of the silicon substrate, (ii) a first etching step for etching the silicon substrate by applying a voltage to the silicon substrate to form narrow etched portions (openings, trenches) that extend in a direction depthwise of the silicon substrate from the etching start patterns while the silicon substrate is immersed in a solution containing fluorine ions, with the silicon substrate used a positive electrode, (iii) a second etching step for accelerating etching of the silicon substrate by increasing a current flowing through the silicon substrate after the narrow etched portions have reached a predetermined depth, to thereby form wide etched portions (openings, trenches) wider than the narrow etched portions at a location deeper than the predetermined depth, and (iv) a hollow passage forming step for forming a hollow passage in the silicon substrate by burying the narrow etched portions after the wide etched portions have reached another predetermined depth.
In the manufacturing method, because the hollow passage formed in the silicon substrate can be fabricated by means of a simple manufacturing process, the silicon device having high reliability can be obtained at a low cost. That is, according to the manufacturing method, because the hollow passage in the silicon substrate, through which a fluid can be passed, may be fabricated by means of the simplified step, its productivity may be improved and further the silicon device may be obtained at a low cost.
In the method of manufacturing the silicon device according to the first or second aspect of the present invention, the etching start patterns may consist of pits formed on the surface of the silicon substrate, a mask formed on the silicon substrate or p-type regions formed by burying p-type materials in a surface of an n-type silicon substrate.
Hereupon, when the etching patterns consist of the pits, the etching patterns can be formed by means of a technique which does not depend on the crystal orientation of the silicon substrate, and further the manufactured silicon device is not affected by the crystal orientation of the silicon substrate. Therefore, the silicon device can be fabricated in any desired shape. In consequence, the fabricated silicon device may have excellent functions and also be small-sized. That is, because the etching start patterns are formed by the technique, such as the reactive ion etching technique or the like, which is not affected by the crystal orientation of the silicon substrate composed of silicon, the etching start patterns can be formed in any desired shapes on the silicon substrate so that the free standing structure fabricated by the continuously performed etching process may have any desired shape. Therefore, the obtained silicon device structure may have an excellent performance and also be small-sized.
When the etching start patterns consist of the mask, the manufacturing step is simplified, and further the structure can be fabricated in any desired shape without being affected by the crystal orientation of the silicon substrate. In consequence, the silicon device, which has excellent functions and is small-sized, can be manufactured at a low cost. That is, because the etching start patterns are composed of, for example, a silicon nitride film which has been patterned by the photolithography technique and is not affected by the crystal orientation of the silicon substrate, they may have any desired shapes on the silicon substrate. In consequence, the free standing structure fabricated by the continuously performed etching process may have any desired shapes so that the fabricated silicon device structure may have an excellent performance and be small-sized. Further, because the initial etching step for etching the silicon substrate is not required when the etching start patterns are formed, the manufacturing process is simplified so that the silicon device may be obtained at a low cost.
When the etching start patterns consist of the p-type regions, the etching patterns can be formed by a technique which does not depend on the crystal orientation of the silicon substrate, and further the manufactured silicon device is not affected by the crystal orientation of the silicon substrate. Therefore, the silicon device can be fabricated in any desired shape. In consequence, the fabricated silicon device may have an excellent performance and be small-sized. That is, because the mask used for the ion implantation to form the etching start patterns is not affected by the crystal orientation of the silicon substrate, the etching start patterns, in which the p-type materials are implanted into the substrate, can be formed in any desired shape on the silicon substrate. In consequence, the free standing structure fabricated by the continuously performed etching process may have any desired shape so that the fabricated silicon device structure may have an excellent performance and be small-sized.
When an n-type silicon substrate is used as the silicon substrate in the method of manufacturing the silicon device according to the present invention, the current flowing through the silicon substrate can be controlled by varying the light intensity or the voltage applied to the silicon substrate while applying light to the silicon substrate in each of the first and second etching steps.
In that case, because the n-type silicon substrate is used, positive holes required for the etching are supplied by applying light. In consequence, the supplying amount of the holes can be controlled by varying the light intensity so that the device having the free standing structure can be fabricated with good accuracy. In the silicon device fabricated in the above-mentioned manner, because the fabricated free standing structure is composed of single crystal silicon, the obtained device may have excellent mechanical properties and high reliability. Further, because the hollow portion below the free standing structure can be formed with a large size, the free standing structure may not stick to the plate-shaped substrate thereunder. In consequence, its yield may be highly improved. Moreover, according to the method of manufacturing the silicon device, the movable portion consisting of the free standing structure can be fabricated by one single step. In consequence, the manufacturing process is simplified so that the silicon device may be obtained at a low cost.
Meanwhile, when a p-type silicon substrate is used as the silicon substrate in the method of manufacturing the silicon device according to the present invention, the current flowing through the silicon substrate can be controlled by varying the voltage applied to the silicon substrate in the first or second etching step.
In that case, because the p-type substrate is used, positive holes required for the etching exist much more in the substrate. In consequence, it is not required to apply light to the silicon substrate so that the etching apparatus may be simplified and further there may be achieved a uniform etching rate in the silicon substrate. Therefore, the silicon device may have high reliability and be obtained at a low cost. Further, because the non-uniformity of the etching due to the distribution of the light intensity is prevented, the etching in the silicon substrate may be uniformly performed so that the silicon device may have high reliability and be obtained at a low cost. Moreover, because the fabricated free standing structure is composed of single crystal silicon in the silicon device manufactured by using the manufacturing method, the obtained device may have excellent mechanical properties and high reliability. Meanwhile, because the hollow portion below the free standing structure can be formed with a large size, the free standing structure may not stick to the plate-shaped substrate thereunder. In consequence, its yield may be highly improved. In addition, according to the method of manufacturing the silicon device, the movable portion consisting of the free standing structure can be fabricated by one single step. Therefore, the manufacturing process is simplified so that the silicon device may be obtained at a low cost.